Field
In some aspects, the following relates to methods and apparatus for decoding video images, and in particular to methods and apparatus for decoding compressed motion compensated video images.
Related Art
There are many video compression techniques and some of these are incorporated in standards. The best known is the MPEG standard, which has had several versions since first conception. One of the most commonly used current compression standards is H.264.
Video data typically comprises a sequence of non-interlaced frames or interlaced fields of video data. Typically, compression techniques will allocate every Nth frame as a reference frame which will be transmitted in its entirety. Frames between the reference frames are each represented by a set of motion vectors which, when allocated to areas in the reference frame, point to areas in an intermediate frame, which are a best match to the respective area in the reference frame. These motion vectors are usually allocated using a block based matching system in which a series of candidate vectors are assigned to each block and tested to see which gives the best match between a block in a reference frame and a block pointed to by the motion vector. Transmission of sets of these motion vectors with a reference frame enables two or more frames to be transmitted in compressed format since the motion vectors comprise less data than the field or frame which they are used to derive. The compressed frames can either be of I, P or B. I pictures do not reference any other pictures and are known as reference pictures. Both P and B pictures reference other pictures, use motion vectors, and residual data encoded in the current picture to reference to areas in the reference picture and use the residual to add to the reference area to correct any differences between the reference area and the current area. These are known as intermediate pictures. B pictures differ from P pictures in that they can reference two previously decoded pictures rather than one.
In most systems, an intermediate frame is reconstructed at the encoding end of a system and difference data between the reconstructed picture and the actual picture is derived. This can be transmitted as correction data for the field or frame represented by the motion vectors to improve quality at a decoder.
In a straightforward compression system, an intermediate picture will be derived only with reference to a reference picture. However, in more sophisticated systems such as H.264 intermediate fields or frames can be derived using motion vectors, which originate from other reference pictures. These are typically referred to as B pictures. In these systems, it is not possible to decode an intermediate picture which is derived from a preceding picture or uses at least some data from a preceding picture until the data for that preceding intermediate frame has itself been decoded.